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Roots of Unity

Roots of Unity: Level 4 Challenges


\[\cos\left( \frac{\pi}{11}\right) - \cos\left( \frac{2\pi}{11} \right)+ \cos\left( \frac{3\pi}{11}\right) \\ - \cos\left( \frac{4\pi}{11} \right)+ \cos\left( \frac{5\pi}{11} \right)=\ ?\]

Given \(z^2+z+1=0,\) find the value of

\[\left(z+\dfrac{1}{z} \right)^2+\left(z^2+\dfrac{1}{z^2}\right)^2+\left(z^3+\dfrac{1}{z^3} \right)^2+\cdots+\left(z^{21}+\dfrac{1}{z^{21}}\right)^2.\]

\[ \large{\dfrac{31-2\delta_{1}}{1-2\delta_{1}} +\dfrac{31-2\delta_{2}}{1-2\delta_{2}}+\dfrac{31-2\delta_{3}}{1-2\delta_{3}}} \]

If \(\large{1,\delta_{1},\delta_{2},\delta_{3}}\) are distinct fourth roots of unity, then evaluate the expression above.

If \(w=e^{2\pi{i}/5}\), find \[\sum_{k=1}^{4}\frac{1}{1+w^k+w^{2k}}\]

If for \(z \in \mathbb{C} \space ,\) \(z+\dfrac{1}{z}=2 \cos 6°.\) Then find the value of \[\left( z^{1000}+\frac{1}{z^{1000}} \right).\]


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